When designing the cell layout for a wireless cellular communication system, it is necessary for the various cells to overlap enough to ensure that there are no "dead" areas in the cellular coverage area between cells. The cell overlap layout requirement generally stems from the system imposed hand-off requirements between cells. A typical requirement consists of two rules: (1) sense the occurrence of a particular trigger signal strength at the receiver, e.g., -70 dBm, and notify the system that a hand-off to a new cell is required to maintain system performance, and (2) find a suitable cell to transfer to by finding another cell with a signal strength that is at least equal to some specific target value, e.g., -65 dBm.
In a free space unobstructed, environment, the shape of a cellular radio coverage area, called a cell, is circular in a plane with a size that is easily calculated. In this environment, it is relatively easy to design a cell layout which provides complete coverage without a substantial degree of overlap between the cells. Typical layout procedures based on the ideal, free-space circularly shaped cell require the designer to either ensure that all cells are touching at a particular signal strength or to ensure that all cells are overlapping at a particular signal strength. A nearly perfect layout of circular cells 5 within a defined geographic region 10 is illustrated in FIG. 1.
While the circular cell model is useful for some situations, in typical indoor environments, the shape of the cell becomes highly distorted due to signal reflections, scattering, and absorption caused by the surrounding structure and various objects within the environment. Sophisticated computer modeling of the indoor environment is used to determine the shape of a cell originating from a given point by calculating signal strength as a function of distance from the center of the cell, i.e., the signal source, with due regard for known environmental characteristics, such as the position and composition of walls, metal supports, wiring, etc.
Conventional site layout software tools, such as WiSE ("Wireless System Engineering" tool), generally include a graphical user interface ("GUI") through which the calculated shape of the cells is displayed. The user is permitted to adjust the position of the cells based on the graphically displayed information to obtain good coverage while complying with handoff requirements. The cells are typically displayed in color using a graded color map or continuous color contours of a cell, where the color grade or the contours are determined as a function of signal strength away from the center of the cell. This allows the various signal strength boundaries and cell overlap to be visualized.
As is apparent, in order to guarantee good handoff operation, the closer the cells are placed together, the greater the overlap. However, a competing design requirement is to maximize cell coverage to reduce the number of cells which are required to cover a given area. While conventional color layout tools, such as WiSE, make it easier for a layout designer to achieve complete coverage, they result in relatively conservative layouts since it is difficult to determine whether the cells are too close together, and therefore, whether more cells than necessary have been used.
Accordingly, it would be advantageous to provide a graphically based wireless cell layout tool which provides an improved visualization of the cell overlap relative to the handoff requirements that both satisfies the system hand-off requirements and at the same time maximizes cell coverage.